Fibromyalgia (FM) is a disease of which the main symptom is systemic pain persistent for 3 months or more and it is known that the disease is triggered by chronic stress, psychological trauma, acute diseases and the like. Because the main symptom, pain, diffuses to entheses, muscles and joints and from extremities to the whole body and continues over a long period of time, the quality of life (QOL) of patients is significantly decreased. The prevalence of the disease is about 2% in Japan, Germany and the United States of America according to epidemiological researches and currently about 2 million people are suffering from the disease in Japan. The marketed drugs that may treat pains accompanied with fibromyalgia so far include pregabalin, duloxetine, milnacipran and the like. However, as the drugs alleviate pain and thus only palliative therapy is available, there is a need for a drug that can radically cure fibromyalgia.
Meanwhile, TSPO (translocator protein 18 kDa) is a protein also referred to as MBR (mitochondrial benzodiazepine receptor) or PBR (peripheral benzodiazepine receptor) and attracts attention as a pharmacological target of stress diseases typically including irritable bowel syndrome (IBS). It is known that TSPO exists at the mitochondrial outer membrane of various cells such as macrophages, microglia and reactive astrocytes and involved in cholesterol transportation and steroid production (see NPL 1). It is believed that under stressed conditions, the amount of neurosteroids, a type of steroids, in the brain changes, the balance between the excitatory and inhibitory signal transduction systems is disrupted, thereby changing the activities in the nervous system, the immune system and the endocrine system and causing various stress diseases. It has been recently reported that expression of TSPO is elevated in monocytes of fibromyalgia patients (see NPL 2) and that expression of TSPO is also elevated in thrombocytes of fibromyalgia patients (see NPL 3).
However, the prior art documents merely disclose an elevated expression of TSPO observed in fibromyalgia patients and do not disclose or suggest that the present compound which has an antagonistic effect of TSPO (see PTL 1) can be an excellent therapeutic agent for fibromyalgia compared to well-known TSPO antagonists.